1. Field of the Invention
The present invention relates to a superconductive magneto-resistive device for a magnetic sensor.
2. Description of the Prior Art
Conventionally, a magnetic sensor which utilizes the Hall effect on magneto-resistive effect in a semiconductor or a magnetic sensor which utilizes the magneto-resistive effect in a magnetic material is widely used for sensing or measuring a magnetic field. The former sensor has a sensitivity capable of sensing a magnetic field of about 10.sup.-2 gausses and the latter one has a sensitivity of about 10.sup.-3 gausses.
However, these conventional magnetic sensors have various disadvantages as follows.
They have relatively large specific resistance R.sub.0 even when no magnetic field is applied to them.
Each variation ratio of resistance to the magnetic field is represented by a parabolic curve having a small coefficient, as shown in FIG. 1 qualitatively. Since a gain .DELTA.R in the resistance is increased proportional to the square of the magnetic flux density B of an applied magnetic field, the gain related to the application of a weak magnetic field of, for example, several tens of gausses is very small and, therefore, a ratio of the gain .DELTA.R to the specific resistance R.sub.0 (.DELTA.R/R.sub.0) is on the order of 1% at the most.
On the contrary, a magnetic sensor with use of the SQUID (Superconductive Quantum Interference Device) which utilizes the Josephson junction is known to have a very high sensitivity capable of sensing a very weak magnetic field of about 10.sup.-10 gauss. Structures of tunnel junction, point contact and micro bridge have been known as the Josephson junction.
However, the magnetic sensor of this type has a quite delicate structure in manufacturing and requires a complicated operation to use it. Namely, it is not practical for general use although it has a very high sensitivity.
In a copending application (U.S. Ser. No. 226,067) which was filed in the name of KATAOKA et al on Jul. 29, 1988 and will be assigned to SHARP KABUSHIKI KAISHA, a superconductive magneto-resistive device is proposed which is comprised of a superconductive material having grain boundaries acting as weak couplings and means for utilizing a change in the resistance of the material caused when a magnetic field is applied thereto.
As shown schematically in FIG. 2, the superconductive material is comprised of superconductive grains 1 and grain boundaries 2 bonding them. These random grain boundaries 2 are considered or supposed to form various weak couplings 3 including tunnel junctions, point contact junctions and micro bridge junctions, as shown by an equivalent network circuit of FIG. 3. In the superconductive phase thereof, individual Cooper pairs can pass freely through weak couplings 3 (Josephson junction) and, therefore, the resistance becomes zero. When a magnetic field is applied to the superconductor, some of Josephson junction 3 are broken thereby and, accordingly, the superconductor has an electric resistance. As a superconductor having grain boundaries, a Y-Ba-Cu-O ceramic superconductor can be used. The critical temperature thereof is about 90 K.
FIG. 4 shows an example of the magnetic sensor system disclosed in the above identified application.
In this system, an elongated rectangular device 4 of (1.times.7.times.0.7 mm.sup.3) which is made of a Y-Ba-Cu-O ceramic superconductive material is prepared and is immersed in liquid nitrogen (77 K). A current is supplied by a power source 9 through a pair of electrodes 5 and 6 formed on respective ends thereof and a voltage between two electrodes 7 and 8 is measured to detect a change in the resistance thereof when a magnetic field B is applied thereto.
FIG. 5 shows the result obtained. As is apparent therefrom, the resistance of the device 4 changes according to the strength I of the applied current and that of the applied magnetic field B. One of the advantages of this system is that the specific resistance of the device is zero in the superconductive phase and another advantage is that the change in the resistance of the device is very steep and, therefore, a very high sensitivity to the magnetic field is obtained.
However, in this system, there is a problem which is that the magnetic sensor senses a magnetic field induced by the current flowing through the device because of the fine sensitivity thereof. In order to avoid this problem, it is desirable to form the superconductive device linearly, as shown in FIG. 7. But, such a linear device induces a magnetic field proportional to the length thereof which causes an error in the measurement of the strength of an external magnetic field to be measured.